Russian president Vladimir Putin has called his United States counterpart Barack Obama to discuss an American diplomatic proposal for Ukraine, the White House said.

It is believed to have been the first direct conversation between the leaders since the US and its European allies began imposing sanctions on Mr Putin's inner circle and threatened to penalise key sectors of Russia's economy.

Russia's reinforcement of troops near Ukraine has brought the total forces there to as many as 40,000, US officials estimated, in a build-up that has increasingly worried Washington in recent days.

The White House noted specifically that it was Mr Putin who called Mr Obama, who is ending a four-country trip in Saudi Arabia and had just returned to his Riyadh hotel after talks with King Abdullah.

Mr Putin called to discuss a US proposal for a diplomatic resolution to the Ukraine crisis, which secretary of state John Kerry again presented to Russian foreign minister Sergei Lavrov at a meeting at The Hague earlier this week.

The US has been pressing Russia to pull back its troops to their Crimean bases and agree to talks with the Ukrainian government with international mediation.

International monitors would go into Ukraine to assure that the ethnic Russian minority there is safe.

"President Obama suggested that Russia put a concrete response in writing and the presidents agreed that Kerry and Lavrov would meet to discuss the next steps," the White House said.

President Obama made clear that this remains possible only if Russia pulls back its troops and does not take any steps to further violate Ukraine's territorial integrity and sovereignty

White House statement

A senior Obama administration official described the call as "frank and direct", and said the next step was the Kerry-Lavrov follow-up discussions to see whether the Russians were serious about diplomacy.

The official also said Mr Obama and Mr Putin discussed the latest version of a "working document" that Mr Kerry and Mr Lavrov have been working on regarding ways to defuse the crisis.

"We have been in close touch with the Ukrainian government in this process," the official added.

In a statement posted on its website, the Kremlin said Mr Putin had cited the Russian justification for its actions toward Ukraine, which has centred on concern for Russian speakers there since the overthrow in February of Ukraine's pro-Russian president.

It said Mr Putin "drew Barack Obama's attention to the continued rampage of extremists who are committing acts of intimidation towards peaceful residents, government authorities and law enforcement agencies in various regions and in Kiev with impunity".

It was in light of this, the statement said, that the Russian leader had proposed actions by "the global community" to restore stability.

Pressure mounts to de-escalate crisis

Mr Obama and European leaders this week piled pressure on Russia to de-escalate the Ukraine crisis in a peaceful way.

In a speech on Wednesday in Brussels, Mr Obama built his case for sanctions against parts of the Russian economy like the energy industry and said NATO would bolster its presence in the alliance's member nations close to Russia.

The White House said Mr Obama stressed to Mr Putin that the US continued to support a diplomatic path in close consultation with the Ukrainian government.

"President Obama made clear that this remains possible only if Russia pulls back its troops and does not take any steps to further violate Ukraine's territorial integrity and sovereignty," the White House said.

Russian deployments on the border with Ukraine include the establishment of supply lines and the fielding of a wide range of military forces, US officials said.

These include militia or special forces units made up of Russian fighters wearing uniforms lacking insignia or other identifying markings, similar to the first Russian forces to move into Crimea during Russia's recent military takeover there.